Electrical utility workers, telephone linemen, appliance installation teams, and other types of workers often find themselves working among or near electrical conductors that could potentially carry large dangerous alternating voltages. Electrical utility workers and telephone linemen, for example, often ply their trades near high voltage power lines. Even in situations where these individuals are not working directly in the vicinity of power lines, they nevertheless find themselves working on or with other electrical conductors that can be charged with potentially dangerous alternating voltages. These may include, for example, electrical and telephone junction boxes, power distribution stations, circuit breaker panels, and even the metal exterior skin of house trailers and other metal buildings. These conductors can become electrified through unintended contact with electrical lines, and the contact may be local or miles away down a telephone line or cable. It is extremely important that such workers be able to determine whether conductors and conducting surfaces are charged with potentially dangerous voltages before working on or coming into contact with the conductors. Failure to do so can result in electrocution and instant death when a worker touches a charged conductor.
One method of determining whether a conductor is carrying a dangerous alternating voltage is simply to measure the voltage directly with a voltmeter connected between the conductor and ground. While this method indeed gives an accurate measurement of voltages present on the conductor, it is nevertheless dangerous and highly undesirable, particularly when measuring alternating voltages greater than about 150 volts RMS. This is because direct contact is required between the voltmeter probe and the conductor. Further, in many situations, such as with electrical power lines, the conductor may be covered with an insulator so that direct contact with a voltmeter probe is not practical.
Attempts to design voltage detectors that detect the presence of a dangerous alternating voltage on a conductor without actually contacting the conductor have been made. U.S. Pat. No. 5,414,344 of Chinn, for example, discloses a non-contact apparatus for sensing energization of high voltage signal lines. Other prior attempts are disclosed, for example, in U.S. Pat. No. 4,818,945 of Bossard and U.S. Pat. No. 5,325,047 of Kempton. The devices and methods disclosed in these prior art patents are plagued with respective problems and shortcomings for use by linemen and others in the field.
Devices such as those disclosed in the above patents fail to meet the needs of telephone line workers wishing to detect the presence of dangerous alternating voltages. For example, none of them address an issue of particular interest to electric utility workers and telephone linemen; that is, the ability to detect potentially dangerous alternating voltages on a conductor while ignoring the presence of known benign voltages. Potentially dangerous alternating voltage usually has a primary frequency of 60 hertz and harmonics up to about 180 hertz. Alternating voltages with frequencies above about 200 hertz are usually not dangerous but nevertheless can be induced in conductors and conducting surfaces by radio transmitter signals, microwave signals, and other ambient transmissions. These higher frequency signals can affect voltage detectors by corrupting their measurements or producing a false alarm. False alarms, in turn, can postpone needed work and cost money while it is determined whether a dangerous alternating voltage is in fact present on a conducting surface to be serviced.
Another benign signal commonly present in telephone circuits is the telephone ring signal. The predominant ring signal used in the United States telecommunications industry is a 20 hertz sine wave signal at about 85 to 105 volts RMS. While such a signal is common, it is nevertheless not considered dangerous by linemen and utility workers. However, detection of such a signal by a voltage detector can corrupt a measurement or indicate the presence of a dangerous condition when, in fact, no dangerous condition exists; i.e., generate a false alarm.
U.S. Pat. No. 5,952,820, owned by the assignee of the present application, is directed to a foreign voltage detector that is an improvement over those of the prior art at the time of its filing. The patent discloses a foreign voltage detector designed to detect and warn of the presence of potentially dangerous alternating voltage levels on conductors such as telephone lines and power cables, metal utility boxes, metal building frames, and other conductors. The detector comprises a capacitive leaf sensor that is charged through capacitive coupling when brought near a conductor carrying an alternating voltage. The charge is converted to a voltage signal that is low pass filtered and notch filtered to remove extraneous signals and telephone ring signals. The filtered voltage signal is then converted to DC, and drives a ten element LED bar graph display to indicate the approximate level an alternating voltage detected on the conductor. A buzzer is provided as an audible alarm of the presence of potentially dangerous voltage. Low pass filters and notch filters are employed to reject signals above a given frequency and also to reject signals at and around the common frequency of telephone ring signals. As a result, a reliable detection of dangerous alternating voltages is provided with fewer false alarms caused by benign voltages.
While the foreign voltage detector of the '820 patent proved successful as a commercial product, certain shortcomings became apparent over time. For example, the circuitry of my prior detector was completely analog, which proved rather expensive to manufacture and also not easily adaptable to changes. Perhaps more importantly, however, it became clear over time that the detector of the '820 patent was susceptible to false alarms that could be triggered by electric fields generated by distant high power transmission lines, when in fact the conducting surface to be worked on carried no dangerous voltage. Also, when using the detector of the '820 patent in confined spaces where it was not readily visible, or in noisy environments, or in at least one case, by a deaf lineman, the visual and audible alarms provided by the detector could not always be seen or heard by a worker.
There exists a need for an improved method and apparatus for detecting and indicating to a worker the presence on electrical conductors of potentially dangerous alternating voltages. Such a method and apparatus should effectively filter out the presence of electrical signals having frequencies above about 200 hertz to filter out stray signals caused, for example, by radio transmitters. Furthermore, such a detector should filter out accurately and effectively signals produced at about 20 hertz that correspond to telephone ring signals so that these signals will not corrupt measurements or produce a false alarm. The improved method and apparatus should be immune to false alarms caused by distant high power transmission lines, should be highly adaptable to changing performance requirements, and should be configured as a small handheld device that can be carried by utility workers and linemen. The apparatus should be convenient, reliable, accurate, and should be economical and feasible to produce in large quantities. It is to the provision of such a method and apparatus that the present invention is primarily directed.